Rules aimed at helping drivers avoid unintentionally backing over children, already overdue, are being delayed again following complaints from automakers that requiring rearview video cameras systems on new cars and trucks would be too expensive.

In a letter to lawmakers, Transportation Secretary Ray LaHood said his department needs more time for "research and data analysis" before it can issue regulations.

"I believe it is important to allot additional time to ensure that the final rule is appropriate and the underlying analysis is robust," LaHood wrote Tuesday. The letter didn't mention the auto industry's concerns.

More than a year ago, the National Highway Traffic Safety Administration proposed requiring improved driver rear visibility in new vehicles, a standard that in most cases would necessitate rear-mounted video cameras with in-vehicle display screens. The regulations were to be phased in, applying to all cars and light trucks by the 2014 model year.

"We're disappointed the government did not take final action today to address this problem, but we understand they are still on a path forward to issuing a rule this year," said Ami Gadhia, an attorney for the Consumers Union, the policy and advocacy arm of Consumer Reports magazine. "We hope that day comes as soon as possible so that rear visibility for all vehicles is improved and needless deaths and injuries are reduced."

The new rear visibility standard was required by a law that Congress passed in 2008 in response to dozens of accidents in which children were backed over. At issue in particular were blind zones in large sport utility vehicles and pickups.

Nearly 300 people are killed and 18,000 injured each year because of back-over accidents, according to NHTSA data. Many occur in driveways and parking lots. Nearly half the deaths involve children under age 5. The elderly also are frequent victims.

Lobbyists for the Alliance of Automobile Manufacturers met with White House officials in December about the proposed rules. Documents brought to the meeting by lobbyists, and posted online by the White House, urged the administration to permit manufacturers the option of meeting the proposed visibility standard through expanded mirrors on vehicles rather than cameras.

The documents pegged the cost to the industry of requiring cameras at $2.7 billion annually. But they said allowing automakers to meet the standard with mirrors instead would cost about $1 billion.

"We had urged the government to explore all the options, including mirrors with a wider field of vision," Gloria Bergquist, vice president of the alliance, said in an interview. "There are a variety of tools that could be used. Certainly the rearview cameras are available, and many people are buying them, so it would give consumers more options in how much extra they paid for these technologies."

Clarence Ditlow, executive director of the Center for Auto Safety, a consumer group, said providing that flexibility is a bad idea, especially since it is people outside the car who are being killed and injured, not the drivers.

"Safety should be for all, not some," he said. "Sure, a mirror is going to be cheaper, but will a mirror get the job done? The answer is no."

Mirrors only work if drivers remember to use them, Ditlow said. But video camera systems that include sensors usually beep or make other audible warnings that alert drivers to the presence of a person behind the vehicle, he said.

The government estimated the video systems would add $159 to $203 to the cost of each new vehicle. But if the vehicle already has a built-in GPS screen, the cost could be as low as $58, Bergquist said.

Automakers are also concerned that the cumulative effect of federal safety regulations is driving up the average price of a new car, which is now about $25,000, she said.

About 45 percent of 2012 model cars have rearview cameras as standard equipment, according to KidsAndCars.org, a safety group that championed the passage of the 2008 law. The cameras are an option on an additional 23 percent of models.

The 2008 law required the government to issue final rules to address back-over accidents by Feb. 28, 2011. LaHood extended that deadline twice previously.

In a majority of back-over accidents in which children were killed, the driver was a family member, according to KidsAndCars.

"The emotional tragedy is off the Richter scale," said Janette Fennell, president and founder of the group.

In a 2005 tragedy, Meredyth Bryant called to her 2-year-old daughter Annabelle to come out from behind a painting contractor's pickup truck in the driveway of the family's home near Richmond, Va., but it was too late. The contractor, who had no way to see the child, backed over Annabelle, killing her.

"These are very preventable accidents," Bryant said. "No one should have to die that way, especially an innocent child right in front of her mother."